A quadrupole mass analyzer is an equipment which is constructed with four electrodes and separates mass of ions passing therethrough by applying electric field to two pairs of electrodes, each pair constructed by connecting two opposing electrodes, and it is the most ideal in the case that a central space thereof has a hyperbolic surface.
FIG. 1 shows an ideal quadrupole mass analyzer with hyperbolic surface.
As shown in FIG. 1, it is ideal for a quadrupole to be manufactured of four parallel metal rods 3 with hyperbolic surface expressed by an equation of X2−Y2=constant. Two pairs of the rods 1 and 2 are made by connecting two opposing rods, and U+V cos(2πft) is applied to one pair thereof and −U−V cos(2πft) is applied to the other pair thereof (herein, U indicates DC voltage, V indicates a peak value of radio frequency (RF) voltage, and f indicates frequency of the RF voltage). When a specific ion is entered into the quadrupole, it moves with oscillation in a direction perpendicular to a proceeding axis. This movement is determined by two differential equations which are called as Mathieu equation. Though an ion with selected mass passes through the quadrupole with a stable movement, ions with different mass are eliminated by collision into the rods as the movement thereof is unstable in which an amplitude of the oscillation is getting bigger.
A circular rod 4, as shown in FIG. 2, can substitute only some central part of the hyperbolic surface, and the more it become distant from the center the more it is different from the ideal hyperbolic electric field. Therefore, split of mass spectrum peak and decrement of resolving power are occurred by nonlinear motion of ions passing through the quadrupole.
FIG. 3 shows a conventional quadrupole mass analyzer constructed with circular rods and an electric connection of each rod. The conventional quadrupole mass analyzer is constructed with a main filter electrode part 6, the length thereof being more than 100 mm, and a prefilter electrode part 7, the length thereof being about 20 mm. Separation between the main filter electrode part 6 and the prefilter electrode part 7 of the quadrupole mass analyzer is about 2 mm. An RF/DC electric source 9 is connected to the quadrupole main filter electrode part 6, to one pair thereof is applied U+V cos(2πft) and to the other pair is applied −U−V cos(2πft) which is opposite phase of the prior. Same positions of the main filter electrode part 6 and the prefilter electrode part 7 of the each pair are connected with a capacitor and RF voltage V cos(2πft), in which the DC voltage U is blocked, is applied thereto. To the quadrupole prefilter electrode part 7 is applied proper DC voltage through about 10MΩ of resistor 11, whereby an ion beam 8 is easily entered into the quadrupole.
The quadrupole prefilter electrode part 7 removes in advance small ions of which mass is less than 30% of the mass of ions passing through the main filter electrode part 6 when the ion beam 9 passes through the inside of the quadrupole prefilter electrode part 7 where only RF voltage is applied.
In a mass spectrometer which analyzes organic samples such as a Gas chromatograph-mass spectrometer (GCMS), in order to prevent the organic materials from adhering to an ion source and a quadrupole mass analyzer, they are heated by a cartridge heater to maintain temperature about 200 to 250° C. Further, in a Residual Gas Analyzer (RGA) used in analysis of vacuum residual gas components, a whole vacuum chamber is heated to about 200° C. so as to drop a background peak. In such cases, a conventional quadrupole mass analyzer made of metal rods is subjected to extreme expansion and contraction by heat, whereby it gradually loses an original assembling accuracy with oxidation of the metal surface, and at the same time its performance is gradually dropped. As such, a conventional quadrupole mass analyzer made of circular metal rods has a difficulty in that a prefilter has to be made separately to a quadrupole main filter and be attached accurately in the same axis of the main filter in parallel to the main filter as well as disadvantages of split of peak and decrement of resolving power by nonlinear motion of ions and gradual drop of a performance.
A first attempt for resolving the difficulty of assembling the four circular rods with accuracy and a problem of gradual torsion by expansion and contraction of metal is disclosed in U.S. Pat. No. 3,328,146 in 1967, in this invention a mandrel with four cylindrical concave is made of Cr—Ni steel or stainless steel and is fitted with a glass tube, and then the glass tube is pumped by vacuum pump while being heated to the temperature in which a glass is deformable. Then, the glass is contracted and adhered to a surface of the mandrel, an integral quadrupole shape is formed within the glass tube by removing the mandrel after the temperature of the mandrel being dropped to a room temperature, the four cylindrical surface inside of the glass is gold-plated and then used as a quadrupole. As a glass tube to be used in this method, a special glass having coefficient of thermal expansion similar to that of steel used as a mandrel has to be used. If the coefficient of thermal expansion is different in a little, the glass is broken in pieces while the temperature is dropped. An integral quadrupole with light weight can be manufactured by such method, however commercial sales thereof was not realized relative to the quadrupole made of four metal rods due to high coefficient of thermal expansion of glass which is similar to steel, low level of gold-plating ability at that time and difficulty of manufacturing.
In 1988, a method for manufacturing an integral quartz quadrupole mass analyzer using a quartz tube instead of a glass tube and a molybdenum mandrel with hyperbolic surface by the same method as the above mentioned method is disclosed by Hewlett-Packard Co. (U.S. Pat. No. 4,885,500). In this method, because it is difficult to gold-plate accurately four hyperbolic surface which are located inside of an integral quartz with narrow space and are about 200 mm in length, a mandrel has four stainless steel plates and four hyperbolic surface electrodes are constructed by the steel plates being pressed to be attached to an inside of the quartz tube when the quartz tube is attached to the mandrel. However, as coefficient of thermal expansion of a quartz tube is different from that of a stainless steel plate, the quartz tube is easily broken and oxidation of the molybdenum mandrel and the stainless steel plates is great when the quartz tube is heated to 1550° C. at which temperature the quartz tube is deformable to have hyperbolic surface and then the temperature thereof drops to a room temperature, there is also a problem that electric charges get accumulated in a concave between the stainless steel plate electrodes thereby deforming hyperbolic electric fields of the electrode portion. Therefore, it is very difficult to manufacture actually a quadrupole by this method.